Microstructure and physical properties of black-aluminum antireflective films

The microstructure and physical properties of reflective and black aluminum were compared for layers of different thicknesses deposited by magnetron sputtering on fused silica substrates. Reflective Al layers followed the Volmer–Weber growth mechanism classically observed for polycrystalline metal films. On the contrary, the extra nitrogen gas used to deposit the black aluminum layers modified the growth mechanism and changed the film morphologies. Nitrogen cumulated in the grain boundaries, favoring the pinning effect and stopping crystallite growth. High defect concentration, especially vacancies, led to strong columnar growth. Properties reported for black aluminum tend to be promising for sensors and emissivity applications.

Figure A3 shows diffraction rings observed by electron diffraction of the 410 nm thick B-Al film.The pattern was indexed using the tool ringGUI from CrysTBox 1 and the Al unit cell parameter a = 4.054 Å 2 , according to the crystallographic information file obtained in the Inorganic Crystal Structure Database 3 .
Figure A4 presents the column-like structure of the 410 nm B-Al film.The red line indicates where the EDX line scan mapping was performed on the cross-section of the lamella.The graph on the right side shows the higher Si content on the fused silica substrate (~ 100 nm), from where the B-Al film starts.Al content is high at the Al grains, with increasing N content when the line crosses the pore at ~220 nm and again close to the top of the film at ~370 -420 nm.After that, the Pt protection layer is present.

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The first line profile shows the top edge of the column with decreasing Al concentration at ~25 nm, where a small pore is present.A slight increase of the Al content is observed around 60-75 nm, where an Al grain is located, and again around 140 nm, when the line profile approaches the big grain on the right side of the image.N content has a slight increase from below 20% to almost 30% at around 25 nm, where the small pore is present.Another slight increase starts at 50 nm until ~75 nm, it stays lower than 15% until the line profile approaches the big grain on the right side of the figure (~140 nm), where the N content in the pore around the grain is higher.Otherwise, the N percentage is lower than 15%, where the Pt layer diffused within the pores of the film.
The oxygen concentration detected in the film comes from the Al oxidation under the air during the lamella preparation and the time the lamella was stored under ambient conditions.The second line profile crosses a big Al grain, followed by a small pore around 100 nm, with the N content increasing exactly where the pore is present.The second grain is on the border with a pore, which is confirmed by the average higher N percentage (~20%) after 100 nm, than before that (~15%).Even though the difference is subtle, the qualitative behavior can still be observed.The third line profile presents similar Al and N content oscillations as the previous lines, with a higher N percentage around the pores and a lower concentration close to the Al crystals.

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Figure A1 presents the R-Al and B-Al films as grown.Transmission electron microscopy

Figure A2 :
Figure A2: Astar orientation map of the R-Al film with a thickness of ~410 nm.

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Figure A1: R-Al and B-Al films and their appearance.

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FigureA2presents the t-EBSD of the 410 nm thick R-Al film, showing that there is no preferred grain orientation of the Al crystals during the film growth.Three regions can be observed in the film: the first one, in contact with the substrate (at the bottom of the figure), contains small crystalline grains; the second one has bigger crystals formed by the coalescence of the smaller ones; the third region has crystalline columns with no preferred orientation.Grain orientation can be identified using the color-coded inverse pole figure shown on the right side of the figure.FigureA3shows diffraction rings observed by electron diffraction of the 410 nm thick B-Al film.The pattern was indexed using the tool ringGUI from CrysTBox 1 and the Al unit cell parameter a = 4.054 Å 2 , according to the crystallographic information file obtained in the Inorganic Crystal Structure Database 3 .FigureA4presents the column-like structure of the 410 nm B-Al film.The red line indicates where the EDX line scan mapping was performed on the cross-section of the lamella.The graph on the right side shows the higher Si content on the fused silica substrate (~ 100 nm), from where the B-Al film starts.Al content is high at the Al grains, with increasing N content when the line crosses the pore at ~220 nm and again close to the top of the film at ~370 -420 nm.After that, the Pt protection layer is present.

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Figure A5 presents three line profiles of one region of the 250 nm B-Al film, shown in Figure 4 in the manuscript.The three line profiles start with around 70% of Al concentration, where the Al crystal is present.

Figure A4 :
Figure A4: Cross section of the 410 nm B-Al film, and the line profile along the film thicknesses, as shown by the red line in the inset at higher magnification.The graph on the upper right corner shows the Al, N, Si and Pt contents on the film along the line profile.